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From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: Flow control with Telebit modems
Message-ID: <3757.27a61e69@hayes.uucp>
Date: 30 Jan 91 01:16:25 GMT
References: <1991Jan29.142326@apollo.HP.COM>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 39

In article <1991Jan29.142326@apollo.HP.COM>, mishkin@apollo.HP.COM
(Nathaniel Mishkin) writes: 

> Suppose I have 2 Telebit modems connected in PEP mode.  Suppose further
> that both modems have correctly established flow control in both directions
> to their respective DTEs.  My question is this:  If one of the DTEs says
> "stop" (i.e., sends an XOFF to the modem or drops RTS, depending on which
> kind of flow control is set), will the modem attached to that DTE send
> any sort of "stop" signal to the remote modem and induce that remote
> modem to say "stop" to its attached DTE (e.g., send an XOFF to the DTE
> or drop CTS)?  ...

I don't know exactly how Telebit does it in PEP, but I assume it is 
something like the way CCITT V.42 does it.  I _do_ know a few things 
about V.42.

In V.42, when a DTE XOFF's a modem (or drops RTS), this stops the 
flow of data from DCE to DTE.  The DCE has several options in this 
case.  It could choose to immediately send an RNR (Receive Not 
Ready) frame to the remote modem, to immediately stop incoming data. 
It could let data back up in the DTE output buffer, and eventually 
force the transmission of an RNR when the buffers fill up.  In 
either case, assuming the flow-off condition persists long enough, 
the remote modem will eventually be told by the local modem not to 
send any more data.  The remote modem might immediately flow-off 
its DTE, or it could also allow its DTE input buffer to fill up, 
eventually causing a flow-off due to the buffer threshold being 
exceeded.  

So, the answer to your question is, yes, modem error control 
protocols in general do have a way to flow off between modems, 
although this mechanism might not be directly invoked by a flow-off 
condition on the DTE-DCE interface.

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-449-8791  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
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